Hi Bender, According to my data, I can confirm that 6G1553 is an RD4, manufactured early 1942 (Serial numbers for 1942 ran from 6G1516 to 6G2942). I understand that from 6G1 to 6G876 the tractor was designated a "Thirty" as cast into the radiator tank top, from 6G877 it changed to RD4.
The SP in your serial number means "Special Parts", I believe that means that special (non standard) gear ratios were installed in that tractor. I have a D2 ser No. 5J2720SP and it was fitted with different ratios for 3rd and 4th gear according to the parts book, I believe for agricultural purposes. It could well be the same for the R4.
As SJ states, many R4's were re-engined with a diesel for economic reasons (probably when the gas engine needed a major overhaul)...well, they were here in UK where gas (petrol) has always been more expensive than in US.
Assuming your engine is a "D4" engine, and has an Eiseman Magneto Model RC-2H, the points gap should be 20 thou, according to my D4 servicemans reference book.

Hope this helps !

Hi, Just realised I made a typo in my earlier post the 6G ser number is an R4 (not RD4, oops !). 5T19WSP looks like a D4 tractor ser No. the W denotes wide gauge as all D4 had the same prefix, unlike e.g. D2 which were 3J narrow and 5J wide gauge. Don't know why the D4 was numbered differently.

Hi, Just realised I made a typo in my earlier post the 6G ser number is an R4 (not RD4, oops !). 5T19WSP looks like a D4 tractor ser No. the W denotes wide gauge as all D4 had the same prefix, unlike e.g. D2 which were 3J narrow and 5J wide gauge. Don't know why the D4 was numbered differently.